This R01 application will investigate a novel signaling pathway, the Hippo pathway, in mammalian heart regeneration. The long-term goal is to develop new treatments for patients with heart failure by generating new cardiomyocytes in the adult heart. The objectives of this application are to gain insight into molecular properties of cardiac tissue that are regulated by Hippo signaling. The central hypothesis is that Hippo signaling is a negative regulatory pathway that prevents cardiomyocyte regeneration in the adult mammalian heart. The specific aims are to determine whether the Hippo and Wnt pathways regulate adult heart repair in a manner similar to the interaction in neonatal hearts. To determine whether an autonomous Wnt Hippo interaction occurs in heart muscle nuclei to control important genes and to investigate whether Hippo functions in a reversible manner. The project is conceptually and technically innovative. Concepts to be tested include new ideas in cardiomyocyte biology and cutting edge genome editing and interrogation technologies to address hypotheses. The significance is high because there are no treatments for heart failure due to cardiomyocyte loss. Devising ways to generate new cardiomyocytes is highly significant.